Battery voltage is difficult to change after your system is built, so choose carefully at the start.
12 volts is simplest and most standard, used in vehicles, RV, and boats. If you want a small simple power system, 12 volts will probably be easiest. You can use 12 volt DC directly in very small systems, adding 120 volt AC with an inverter.
24 volts battery systems have some technical advantage. SYSTEM SIZE: If you think you will have more than 1000 watts of solar modules, consider 24 volts. A technical advantage is that 24 volt wires can run longer distances. If you must place solar modules over 100 feet from the house for adequate sun exposure, or if your wind generator or hydro-power is over 300 feet away, choose 24 volts. Most of your power will be changed to 120 volt AC power. Voltage converters are available to run 12 volt DC equipment from 24 volt batteries.
48 volts has great advantage if longer wire run is unavoidable to reach the only good solar location, up to 400 feet from the house, or hydro turbine 1,000 feet from the house. Presently there is limited selection of 48 volt charge controls and inverters, but these are the best quality, well suited for larger power systems. Voltage converters are available to run 12 volt DC equipment from 48 volt batteries.